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ENGINEERS AT the University of Washington have developed a method for safely charging a smartphone wirelessly from across the room using lasers.

Published in the study 'Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable & Ubiquitous Technologies', the research found that a narrow, invisible beam from a laser emitter can deliver a charge to a mobile device sitting across a room.

The prototype can even potentially charge a smartphone as quickly as a standard USB cable, the engineers claimed.

To design and build this technology, the team mounted a thin power cell to the back of a smartphone, which charges the smartphone using power from the laser.

The team also custom-designed safety features, including a metal, flat-plate heatsink on the smartphone to dissipate excess heat from the laser, as well as a reflector-based mechanism to shut off the laser if a person tries to move in the charging beam's path.

"Safety was our focus in designing this system," said the study's co-author, associate professor, Shyam Gollakota.

"We have designed, constructed and tested this laser-based charging system with a rapid-response safety mechanism, which ensures that the laser emitter will terminate the charging beam before a person comes into the path of the laser."

The study's other co-author, assistant professor of physics and electrical engineering Arka Majumdar, said that in addition to the safety mechanism that quickly terminates the charging beam, the platform includes a heatsink to dissipate excess heat generated by the charging beam.

"These features give our wireless charging system the robust safety standards needed to apply it to a variety of commercial and home settings," he added.

The charging beam is generated by a laser emitter that the team configured to produce a focused beam in the near-infrared spectrum. The safety system that shuts off the charging beam centres on low-power, harmless laser "guard beams", which are emitted by another laser source co-located with the charging laser-beam and which "surrounds" the charging beam.

Custom 3D-printed "retroreflectors" placed around the power cell on the smartphone reflect the guard beams back to photo-diodes on the laser emitter. The guard beams deliver no charge to the phone themselves, but their reflection from the smartphone back to the emitter allows them to serve as a "sensor" for when a person will move in the path of the guard beam.

The researchers said they designed the laser emitter to terminate the charging beam when an object, such as part of a person's body, comes into contact with one of the guard beams. The blocking of the guard beams can be sensed quickly enough to detect the fastest motions of the human body, which the professors said is "based on decades of physiological studies".

"The guard beams are able to act faster than our quickest motions because those beams are reflected back to the emitter at the speed of light," added Gollakota.

"As a result, when the guard beam is interrupted by the movement of a person, the emitter detects this within a fraction of a second and deploys a shutter to block the charging beam before the person can come in contact with it."

The beam charges the smartphone via a power cell mounted on the back of the phone. A narrow beam can deliver a steady two watts of power to 15 square-inch area from a distance of up to 4.3 metres, or about 14 feet.

But the emitter can be modified to expand the charging beam's radius to an area of up to 100 square centimetres from a distance of 12 metres, or nearly 40 feet. This extension means that the emitter could be aimed at a wider charging surface, such as a counter or tabletop, and charge a smartphone placed anywhere on that surface.

"The beam delivers charge as quickly as plugging in your smartphone to a USB port," concluded co-lead author and doctoral student in electrical engineering Elyas Bayati. "But instead of plugging your phone in, you simply place it on a table."

The researchers believe that the laser charger's safety and dissipation features could enable wireless, laser-based charging of other devices, such as cameras, tablets and even laptop computers in the future. µ